Multicarrier modulation has been gaining popularity over single carrier modulation for the transmission of wireless digital signals in a radio communication system. Multicarrier modulation offers increased flexibility over a single carrier modulation by providing opportunities to optimize transmission bandwidth. In a multicarrier modulation scheme, such as multicarrier quadrature amplitude modulation (MCQAM), multiple subcarriers are employed to transmit a particular bit or symbol stream. Typically, the bit stream is divided into multiple parallel bit streams, corresponding in number to the subcarriers. The subcarrier corresponding to each bit stream is modulated by quadrature amplitude modulation (QAM). The modulated subcarriers are then combined, such as by multiplexing, to generate a composite signal. The use of MCQAM reduces the need for system equalization and provides opportunities for enhancing total data throughput, or overall system performance.
In a typical wireless communication system, the presence of noise may affect the receipt and processing of a communicated signal. The impact of such noise varies depending on the environment, which may include factors such as the presence of interfering communications (active interference), passive interference within the communication signal path, and the like. Most sophisticated digital communication systems make some provision to mitigate the effects of noise on communications. For example, a digital signal may be encoded with error correction information, which is used to provide a level of error mitigation. As known, error encoding involves the addition of redundancy bits to encoded data in order to provide for error correction at the receiving end of the signal. As required, different portions of a data stream may be provided with different levels of error protection by using error correction codes with differing coding rates.
While the use of error correction codes provides for mitigation of channel induced errors, the error correction codes typically account for a substantial portion of the transmitted data. As a consequence, additional processing is typically required for encoding and decoding the error correction codes. While, the burdens introduced by correction coding are generally accepted as a necessary to provide data transmission with acceptable error rates, commonly used techniques, such as convolutional encoding and decoding require the addition of computational resources at the transmit and receive ends of the communicated signal, thereby increasing the cost of the overall system.
While, it has been suggested that providing different error rates at different portions of a data stream is a desirable form of error correction, it has likewise been noted that providing different power distribution levels at different portions of a data stream is an alternative form of error correction that may avoid the need of adding error correction information to the transmitted data. Implementation of such a scheme must necessarily take into consideration the spectral permutation of subchannels (i.e., subchannel-to-subchannel interference). It would be extremely advantageous therefore to provide a time domain source matched multicarrier quadrature amplitude modulation method and apparatus that is insensitive to subchannel permutation.